1. Field of the Invention
The present invention is generally related to selective seeding or activation of metal interconnections patterned on polyimide dielectric surfaces for the electroless deposition of metal and, more particularly, to the elimination of bridging or shorting during electroless plating of a thin layer of nickel or cobalt or other metals or alloys on patterned metal interconnections on polyimide dielectric layers.
2. Description of the Prior Art
Electroless metal deposition involves generating a pattern of a suitable catalyst on a substrate and then depositing metal on that pattern. In a typical electroless plating process, an insulating layer of a non-conductive material such as a polyimide is coated on a substrate, followed by the application of a layer of photoresist. The photoresist is then exposed to a radiation source to generate a pattern. After exposure, the photoresist is developed using conventional methods. Metal, usually copper (Cu), is then deposited in the patterned areas to form metal interconnections. The structure is then immersed in an activating solution which contains a catalyst. The catalyst is generally a colloidal solution containing a noble metal such as palladium, which acts as a seed to promote the deposition of further metal on the metal pattern.
The most commonly used activating solutions are palladium acetate or a palladium salt or palladium sol from palladium chloride and stannous chloride. The immersion usually lasts five to fifteen seconds and is performed at room temperature. During the immersion, the surfaces of the metal interconnections are activated, which means that they will readily plate when immersed in an electroless plating solution. The activating metal-reduced surface is then subjected to an electroless metal deposition bath, which is catalyzed by the reduced activating metal, and an electroless metal deposit is obtained on the metal interconnection surfaces. The metal deposit is typically nickel (Ni) or cobalt (Co) or alloys of nickel or cobalt and serves to protect the metal interconnections from oxidation.
The most important functional requirement of the seeding process is to have minimum chemical interactions between the non-conductive substrate or resist surface and the activating metal ions. Any residual metal ions or metallic colloidal particles on this surface could act as a seed for metal deposition on that surface as well as on the seeded metal in the electroless bath. Any such metal contamination and deposition on this insulating surface is deleterious to the electrical performance of the dielectric layers due to shorting, bridging or reduction of breakdown voltage which may be caused.
For example, U.S. Pat. No. 4,262,085 discloses a process for preparing metal patterns on insulating carrier materials in which undesired metal deposition is eliminated. The method involves applying a photosensitizer to the carrier and exposing the carrier to a radiation source to form a pattern. Metal seeds formed on the surface of the carrier are augmented with an aqueous palladium-salt solution, followed by the deposition of a Ni layer. The Ni layer is then treated with a solution containing a complex forming agent for Cu ions. A Cu layer is deposited by means of a currentless coppering bath.
In recent years, polyimide materials have become of substantial interest in the area of electrical component fabrication and for microelectronic components, in particular. Such materials exhibit a desirable combination of high temperature stability and low dielectric constant properties to a unique degree. Further, such materials are easily worked and exhibit a high degree of adhesion with metal layers deposited thereon by, for example, vapor deposition. Polyimide can also be deposited on metal, as well, and can be sensitized to function as a permanent resist. Once cured, the desirable properties of the polyimide are retained and a resist layer can be left as a permanent part of the device structure. In devices which include a polyimide substrate, layer or resist, copper is often the metal of choice for forming conductive patterns as described above.
However, while polyimide materials are extremely desirable, the properties which allow improved adhesion with metals appear to cause a high degree of unintended activation or seeding of the polyimide surface with consequent metal deposition during electroless metal deposition processes. This unwanted activation or seeding decreases manufacturing yields of fabrication processes and reduces performance of components as described above.
As an example of prior attempts to achieve electroless plating of copper deposited on polyimide without seeding of the polyimide, U.S. Pat. No. 4,770,899, to Zeller teaches activation of both the copper and the polyimide with palladium chloride followed by selective deactivation of the polyimide surface with a sodium hydroxide solution. However, the combination of general activation and selective deactivation requires additional process steps and introduces additional process variables. It has also been found that this technique is not optimally effective for the avoidance of seeding and the formation of shorts between conductors in extremely fine patterns. Further, as will be discussed in greater detail below, the presence of chloride ions tends to increase the susceptibility of the copper to corrosion to a degree sufficient to reduce manufacturing yields and product reliability. This corrosive effect of palladium chloride as a seeding solution has particularly severe consequences in electronic devices having extremely fine conductor patterns.